Basilar Membrane

The human cochlea transforms sound waves into electrical signals in the acoustic nerve fibers with high acuity. This transformation occurs via vibrating anisotropic membranes (basilar and tectorial membranes) and frequency-specific hair cell receptors. Frequency-positions can be mapped within the cochlea to create a tonotopic chart which fits an almost-exponential function with lowest frequencies positioned apically and highest frequencies positioned at the cochlear base (Bekesy 1960, Greenwood 1961). To date, models of frequency positions have been based on a two-dimensional analysis with inaccurate representations of the cochlear hook region. In the present study, the first three-dimensional frequency analysis of the cochlea using dendritic mapping to obtain accurate tonotopic maps of the human basilar membrane/organ of Corti and the spiral ganglion was performed. A novel imaging technique, synchrotron radiation phase-contrast imaging, was used and a spiral ganglion frequency function was estimated by nonlinear least squares fitting a Greenwood-like function (F = A (10ax - K)) to the data. The three-dimensional tonotopic data presented herein has large implications for validating electrode position and creating customized frequency maps for cochlear implant recipients.

Auditory sensory outer hair cells are thought to amplify sound-induced basilar membrane vibration through a feedback mechanism to enhance hearing sensitivity. For optimal amplification, the outer hair cell-generated force must act on the basilar membrane at an appropriate time at every cycle. However, the temporal relationship between the outer hair cell-driven reticular lamina vibration and the basilar membrane vibration remains unclear. By measuring sub-nanometer vibrations directly from outer hair cells using a custom-built heterodyne low-coherence interferometer, we demonstrate in living gerbil cochleae that the reticular lamina vibration occurs after, not before, the basilar membrane vibration. Both tone- and click-induced responses indicate that the reticular lamina and basilar membrane vibrate in opposite directions at the cochlear base and they oscillate in phase near the best-frequency location. Our results suggest that outer hair cells enhance hearing sensitivity through a global hydromechanical mechanism, rather than through a local mechanical feedback as commonly supposed.

The cochlear basilar membrane (CBM) contains inner hair cells and outer hair cells that convert sound waves into electrical signals and transmit them to the central auditory system. Cochlear aging, the primary reason of age-related hearing loss, can reduce the signal transmission capacity. There is no ideal in vitro aging model of the CBM. In this study, we cultured the CBM, which was dissected from the cochlea of the C57BL/6 mice 5 days after birth, in a medium containing 20 mg/mL, 40 mg/mL, or 60 mg/mL D-galactose (D-gal). Compared with the control group, the levels of senescence-associated β-galactosidase were increased in a concentration-dependent manner in the CBM of the D-gal groups. In addition, levels of the mitochondrial superoxide and patterns of an age-related mitochondrial DNA3860-bp deletion were significantly increased. The ATP levels and the membrane potential of the mitochondrial were significantly decreased in the CBM of the D-gal groups compared with the control group. Furthermore, in comparison with the control group, damaged hair cell stereocilia and a loss of inner hair cell ribbon synapses were observed in the CBM of the D-gal groups. A loss of hair cells and activation of caspase-3-mediated outer hair cell apoptosis were also observed in the CBM of the high-dose D-gal group. These insults induced by D-gal in the CBM in vitro were similar to the ones that occur in cochlear natural aging in vivo. Thus, we believe that this is a successful in vitro aging model using cultured CBM. These results demonstrate the effects of mitochondrial oxidative damage on presbycusis and provide a reliable aging model to study the mechanisms of presbycusis in vitro.

EMILIN family extracellular glycoproteins in the basilar membrane (BM) have so far only been identified in the mouse. It may influence the stiffness and extracellular filamentous architecture essential for cochlear frequency tuning (Russell et al., 2020, Science Advances, 6, eaba2634). In our study, we explored whether elastic components, such as EMILIN-2 (elastin microfiber interface-located protein 2) and elastin, are also present in the human BM. In addition, we analyzed the fine morphology of the BM and tympanic covering layer (TCL) at different frequency locations using light, transmission electron, and scanning electron microscopy (LM, TEM, and SEM) in well-preserved human cochlear specimens. Tonotopic estimations were made from model extrapolations using synchrotron radiation phase-contrast imaging (SR-PCI) and 3D reconstruction of matched cochleae with delineated octave bands constructed using Greenwood's formula (Greenwood, 1961, Journal of the Acoustical Society of America, 33, 1344). Our immunohistochemistry and gene RNA sequencing (RNA-seq) data support that EMILIN-2 and elastin are expressed in the human BM. Expression pattern of both proteins that influence BM architecture vary along the frequency range. TEM analysis suggests that TCL cells are actively involved in the deposition of the amorphous substance and fibrillary network, conceivably serving to assemble extracellular and elastic constituents along the BM. Thus, our findings suggest that TCL cells may play an important functional role in human cochlear tuning, particularly at low frequencies potentially linked to our remarkable speech and music perception.